In this paper, Silvaco TCAD is used to establishe graphene model, and the experimental bipolar curves of graphene with different doping degrees are reproduced. The simulation results show that applied voltage of top-gate can change the built-in potential between graphene and silicon by adjusting the doping of graphene, so as to enhance the gain of graphene-silicon photodetector. This is consistent with the gain enhancement of the ionic gel gated graphene-silicon photoconductive photodetectors fabricated experimentally in the infrared band. This work has guiding significance for the gain optimization of infrared photodetectors.